Metallization is the process of depositing metal material on the surface of a substrate. Electroplating and other forms of electro-deposition are commonly used metallization techniques to form electrical conductive contacts or protective coatings. For example, electroplating is used in ultra large-scale integration (ULSI) to provide multiple levels of copper or copper alloy metallization.
Metallization is also used in the fabrication of optoelectronic devices such as transparent thin film transistors (TFT), flat panel displays, light-emitting diodes (LED), photovoltaic cells, and electrochromic windows to provide interconnects for transparent device-to-device integration. In the case of electrochromic window fabrication, substrates are typically semiconductors or transparent conductive oxides (TCO), e.g. zinc oxide, indium-tin oxide, and fluorine-doped tin oxide. Furthermore, electroplating is used in the fabrication of power electronics for the metallization of ceramic substrates.
The fabrication of thermoelectric devices or device components also requires metallization on semiconductor substrates. Thermoelectric devices are uniquely advantageous in heat removal and energy harvesting applications because they are free of moving parts, acoustically silent, and they can be integrated into microelectronic devices. Recent advances have greatly improved the thermoelectric figure-of-merit (ZT) of nanostructured thermoelectric alloys. In particular, nanostructured p-type bismuth antimony telluride has achieved a peak figure-of-merit (ZT) of about 1.4 at 100° C.
However, these material property advances have not fully translated into better overall performance in the thermoelectric devices due at least in part to variations in the thermal and electrical contact resistances between the nanostructured alloy substrates and the metallized electrodes. A poorly formed contact generates localized Joule heating effects and leads to a non-uniform current distribution which lowers an effective figure-of-merit (ZTeff) for the thermoelectric device from that of the thermoelectric material.
Generally, in the fabrication of electronic devices, thermoelectric devices, and other metallization processes, desirable electrical and thermal contact properties are highly correlated with a uniform deposition of the metallic atoms on the substrate which creates a strong adhesion and an effective interface between the deposited metal layers and the substrate. In particular, the process of electroplating metal depends on a nucleation process, which is determined by the formation energy, excess energy, and internal strain energy of the phase transition during metallization.
Direct electroplating on smooth, low-roughness, or hydrophobic surfaces of glass, semiconductor, or ceramic substrates is difficult because the target surface has low surface energy or poor wettability, which leads to a relatively high excess energy for electroplating nucleation. As a consequence, scattered and irregular grains of metal grow on a small number of nucleation sites, causing poor interfacial adhesion and large surface roughness. A further consequence of the scattered and irregular grain formation is that strain energy, which is caused by a different atomic arrangement between two adjacent metallization layers, increases with increasing overall metallization thickness, and can sometimes cause metallization layers to spontaneously peel off.
Furthermore, for many applications, metallization is desired on only a portion of the substrate surface, e.g., to form an electrical contact at a specific location. Here, additional processes are typically employed prior to the electroplating process to achieve a selective metallization. For example, a patterning process can be used to form a masking pattern layer on a selected region or regions on the surface of the substrate.
In one commonly used approach to selective metallization, photolithography is employed to create a patterned photoresist layer on the substrate. The exposed regions (the portions not covered by the photoresist mask) can then be etched to create additional surface roughness (or simply to expose the area for metal deposition). Metal can then be sputtered onto the exposed (and etched) regions. These processes improve both the adhesion and electrical conductivity of the primary structure constructed by the subsequent electroplating process. Following the electroplating process, an additional chemical mechanical polishing process can be used to remove any surplus metal and planarize the entire surface. Finally, the photoresist is removed.
Although photolithography and other similar techniques can achieve selective metallization with a high degree of precision, these techniques require costly specialized equipments and can significantly hinder device production rates.
There exists a need for better metallization techniques, especially techniques that can be used on smooth, low-roughness or hydrophobic substrates to achieve high quality metal layers with strong adhesion. Metallization techniques that can achieve selective metallization without the complexity of photolithography would also satisfy a long felt need in the art.